1. Field of the Invention
The invention relates to an endoprosthesis for the femoral part of a hip joint wherein the prosthesis stem is provided with a prosthesis collar supported on a resected bone surface and with a clamping cone which supports the attachable joint ball. At least one screw which grips the prosthesis in the manner of a tensioning stay passes through a lateral bore, oriented obliquely downward and outward in the femur.
2. Discussion of the Background
A tensioning stay endoprosthesis of the above type is disclosed in German Offenlegungsschrift DE-OS 3,522,692.Al. However, the tensioning screw of the prosthesis stem was already described in the publication "Die Zuggurtung-Huftendoprothese" [The Tensioning Stay Hip Joint Endoprosthesis], Arch. Orthop. Unfall-Chir. [Archives of Orthopedic Accident Surgery, Vol. 86, (1976) pages 1 to 14. As indicated there, a relatively short stem is driven into the femur (page 11, paragraph 7 and page 9. FIG. 6), since the stem is given only subordinate significance. The stem, which has a broad, medial contact surface and a longitudinal groove, serves only to transmit relatively small forces.
However, for medical technology reasons, the tilting effect occurring in stem-equipped prostheses must not remain unconsidered. Under permanent load, the thick, non-elastic tip of the stem exerts direct pressure against the relatively soft inner wall of the bone. Moreover, the flow of forces must be diverted from the lower end of the prosthesis stem into the bone, thus inducing locally highly concentrated tensions in the bone. This initially results in a constriction of the marrow cavity due to solidification of the spongiosa and in thicker bone walls. Then the tip of the prosthesis stem seats itself on this bone plug in the marrow cavity so that the flow of forces is now directed almost only through the prosthesis into the plug. The result could be regression of the bone wall above the plug and, in the worst case, loosening and breakage of the prosthesis and the bone.